RE: [Pythoncard-users] greedy events / double buffering?

[Sean D. <se...@tr...>]

Alex,=20

=20

I figured out the first problem - I was only responding to
on_*_mouseMove events, not on_*_mouseDrag.

=20

With the second problem.. I need to use clear() so that I can drag
primitives around the canvas (not draw with them like doodly.py does,
but rather arrange their position). Eventually I'd like to use bitmaps
for my nodes, but right now I'm just using drawRectangle() calls.

=20

I put Freeze() and Thaw() around the code that renders the nodes at
their positions:

=20

    def Render(self):

        if hasattr(self,"app"):

            self.components.GraphCanvas.Freeze()

            self.components.GraphCanvas.clear()

            for node in self.app.node_list:

                pos =3D node.pos

                self.components.GraphCanvas.drawRectangle(pos,(10,10))

            self.components.GraphCanvas.Thaw()

=20

Pretty simple stuff.. Except then I had the following interesting
side-effects:

=20

* Before I added the Freeze()/Thaw() pair, the on_*_mouseDrag event
would only fire once for every mouse movement. Now it just rapidly fires
once the button is down, without moving the mouse.

* Whereas without Freeze()/Thaw() the interface blinks as I drag a node,
with the pair, it doesn't update at all during dragging. I have verified
that the above code is evaluating, though.. After I mouseUp, it draws
everything, with a very noticable single blinking of the whole canvas
which I assume is the buffer blit not being in sync?

=20

So it appears these two problems are very interrelated.. My theory is
that for some reason the mouseDrag event is firing uncontrollably, and
because I redraw the canvas on each drag update, I get a bunch of
recursive Freeze()/Thaw() calls that don't finish popping (and showing
the buffer) until I release the mouse button. It smells like there might
be a forced mouseDrag event firing in wx when Thaw() is called. Anyone
have experience with this issue?=20

=20

I tried a really dumb workaround - before I call Freeze(), I set a class
member (self.thawing) to True, and then when a mouseDrag fires, I check
to see if self.thawing is true, and if it is, I set it to False and
early return, else it updates some data and redraws the canvas:

=20

    def on_GraphCanvas_mouseDrag(self, event):

        if self.thawing:

            self.thawing =3D False

            return

        cur_pos =3D event.position

        if self.drag_mode and len(self.app.selected)>0:

            delta_pos =3D (cur_pos[0] - self.last_mouse_pos[0], =
cur_pos[1]
- self.last_mouse_pos[1])

            self.app.AddRelativeNodePosition(self.app.selected,
delta_pos)

        self.last_mouse_pos =3D cur_pos

        self.Render()

=20

This seems to work, and I can drag my boxes around and it refreshes (and
I totally realize this isn't thread safe)... The remaining problem being
that I now get that sync blinking on every frame. So how do I stabilize
the blit sync? Is the sync even the problem?

=20

I would attach the full source/resource files in this email, but I'm not
sure what the policies are on this list. Is that kind of think ok?

=20

Thanks,

Sean

=20

=20

  _____ =20

From: Alex Tweedly [mailto:al...@tw...]=20
Sent: Tuesday, May 17, 2005 3:23 PM
To: Sean Dunn
Cc: pyt...@li...
Subject: Re: [Pythoncard-users] greedy events / double buffering?

=20

Sean Dunn wrote:=20

I've been using PythonCard for simple interfaces for about a year now.
Recently I decided to try something much more complex - implementing a
custom interface inside a BitmapCanvas. There are two problems I'm
facing at the moment:

=20

* I can't seem to fire a mouseDown event without blocking mouseMove
events until I get a mouseUp event. This basically means I can't click
and drag something interactively inside the BitmapCanvas.

There shouldn't be any problem doing this. Can you be more specific
about the problem ?
Or maybe look at the resourceEditor, which does this kind of thing in
both mouseDown/mouseDrag and in doResize



* I can't figure out how to double-buffer the BitmapCanvas. I'd like to
do a clear() and then redraw my interface without causing the canvas to
blink. Is there a way to do some kind of buffer toggling from inside
PythonCard?

=20

I think you should be able to get what you want from Freeze() and
Thaw().=20
(though in general you ought to avoid clear() and redraw() if possible
.... why do you need to do that ?)



I'd much appreciate if anyone has some sage advice.

=20

By the way, what I'm really trying to do is create a simple interface
for creating node graphs: the kind you see in 2D graphics compositing
programs/visual programming languages where you can arrange node
operators and interconnect them to form a data flow. If anyone knows of
a free Python (or even C++) API for doing this, let me know!

=20

Don't know of one, sorry. There is something along those lines in the
Object Graphic Library (part of wxPython) =3D- see the wxPython demo
(under miscellaneous / OGL - night give you a start towards what you
want.




--=20
Alex Tweedly       http://www.tweedly.net